1. Field of Invention
The present invention relates to a film deposition method. More particularly, the present invention relates to a film deposition method for balancing stress on an optical substrate.
2. Description of Related Art
As data communication via the Internet increases rapidly, the backbone optical cable capacity needs to be upgraded. The requirements for optical components are also demanding. For example, more than one hundred films (thicker than 20 μm) are deposited on a substrate in DWDM filter. Issues previously not of concern become important in manufacturing optical component as the number of film layers increases.
When a DWDM (Dense Wavelength Division Multiplexing) filter is manufactured, all optical films are deposited on a substrate and then the substrate is divided into a desired size. For example, when a round substrate (diameter=90 mm) is employed in the manufacture of DWDM filters, hundreds of films are deposited on the substrate. When the substrate deposited by films is divided, the films peel off or the substrate collapses.
FIG. 1 illustrates the possible results after an optical component is deposited. When thin films are deposited on a substrate, a tensile force may be generated to buckle the substrate or a compressive force may be generated to cause bumps or bubbles on the films. Both the tensile and compressive forces are harmful to process. Taking DWDM optical filter for example, the films peel off or the substrate collapses due to film stress when the substrate deposited by films is divided.
For the forgoing reasons, there is a need to solve the film stress issue when hundreds of films are deposited on an optical component.